Two new species, Pentathyone kojinensis n. sp. and Pseudocnus pawsoni n. sp. belonging to the families Sclerodactylidae and Cucumariidae are described. In addition, four species, Thyone bicornis, Thyone fusus chinensis, Thyone micra, and Molpadia changi, are reported as new records from Korea. Pentathyone kojinensis n. sp. closely resembles Cucumaria mirabilis Theel and C. constrica Ohshima in ossicle form, but the latter two species are clearly distinguished from the new species in having the posterior processes of the calcareous ring in several fragments, rather than stout and entire. Pseudocnus pawsoni n. sp. is similar to Cucumaria koraeensis Ostergren in general features and ossicle morphology though it has not biscuit-shaped plates. However, Cucumaria koraeensis has ten tentacles of the same size, unlike Pseudocnus pawsoni n. sp. in which the two ventral tentacles are much smaller than the remaining eight.